In the past few years, according to the development of genetic engineering techniques, the introduction of genes encoding insect-resistant substances into plants by genetic techniques can enhance the insect-resistance of the transgenic plants. The insect-resistant substances include the toxic crystalline protein, protease inhibitor and the like produced by Bacillus thuringiensis.
Sporamin is a storage protein enriched in the tuberous roots of sweet potato, which was first purified by Maeshima et al. (Maeshima et al., "Characterization of major proteins in sweet potato tuberous roots." Phytochemistry, Vol. 124. pp. 1899-1902, 1985). The amount of Sporamin is about 60% to 80% of the total soluble proteins of sweet potato. Sporamin is present most in tuberous roots, little in other tissues and organs (Hattori et al., "High-level expression of tuberous root storage protein genes from sweet potato in stems of plantlets grown in vitro on sucrose medium." Plant Mol. Biol. Vol. 14, pp. 595-604, 1990). In 1989, Hattori et al. isolated the sporamin gene from the cDNA library of tuberous roots of sweet potato (Hattori et al., "Structural relationship among the members of multigene family coding for the sweet potato tuberous roots storage proteins." Plant Mol. Biol. Vol. 13, pp. 563-572, 1989).
Besides the function of storing nitrogen source, the amino acid sequence of sporamin predicted from the cDNA thereof has been found to have certain homology to Kunitz type trypsin inhibitors of Leguminosae plants (Hattori et al., "Sucrose-induced expression of genes coding for the tuberous root storage protein sweet potato in leaves and petioles." Plant Cell Physiol. Vol. 32, pp. 79-86, 1991). However, Hattori et al. suggested that sporamin has no trypsin inhibitor activities (Hattori et al., 1989).
Moreover, with regard to the relationship between sweet potato sporamin gene and trypsin inhibitor activities was discussed by Chen, Jen-chin in "The research in the genome of sweet potato," Dept. of Botany, National Taiwan University, Master thesis. pp. 1-122 (1994). It is suggested in the article that sporamin might possess trypsin inhibitor activity.
The purpose of the present invention is to find appropriate genes which can be transformed into plants and provide a method of controlling pests. According to the present invention, it is found that sporamin has insect-resistant ability, so it is possible to use sporamin to control pests.